Recently solar photovoltaic systems tend to employ distributed micro inverters (micro-inverters). Micro-inverters may provide maximum power point control for each photovoltaic assembly, such that each assembly can generate maximum energy, thereby improving the performance of the photovoltaic system. The micro-inverters may also have alternating current (AC) low voltage outputs, rather than a direct current (DC) high voltage output from a centralized inverter system, so that security and efficiency can be improved.
FIG. 1 is a simplified internal block diagram of a three-phase micro-inverter in the prior art. As shown, a three-phase micro-inverter 101 includes three single-phase inverter circuits 102, three DC terminals and one three-phase AC terminal (not shown). The three-phase micro-inverter 101 is coupled with three independent solar photovoltaic assemblies 103 via the DC terminals, and couples the three single-phase inverter circuits 102 to a three-phase AC cable via the three-phase AC terminal, with each single-phase inverter circuit 102 being coupled to one phase of a commercial three-phase AC power grid 104.
In FIG. 1, the three single-phase inverter circuits 102 in the above three-phase micro-inverter 101 are typically identical. Each single-phase inverter circuit 102 has one DC input, such that each three-phase micro-inverter 101 is coupled to three solar photovoltaic assemblies 103 via three DC terminals. Each single-phase inverter circuit 102 also has one single-phase AC output, and is coupled to the neutral wire N and one of the live wires L1, L2, or L3 of the three-phase AC power grid 104. As such, the three single-phase inverter circuits 102 are coupled to live wire L1/neutral wire N, live wire L2/neutral wire N, and live wire L3/neutral wire N of one three-phase AC power grid 104, respectively. Each three-phase micro-inverter 101 is coupled to the three-phase AC power grid 104 via the three-phase AC terminal. As described above, each single-phase inverter circuit 102 converts the AC current generated by a connected solar photovoltaic assembly 103 into a single-phase AC current. Since the single-phase inverter circuits 102 are coupled to the three phases of the three-phase AC power grid 104 respectively, the single-phase inverter circuits 102 generate AC currents with phases matching those of the voltages of the three-phase AC power grid 204.
FIG. 2 is a simplified schematic diagram of a three-phase micro-inverter coupled to a three-phase AC power grid in the prior art. As shown, each three-phase micro-inverter 101 is coupled to the three-phase AC power grid 104 in the same way, and the AC outputs A, B, C of each three-phase micro-inverter 101 are coupled to the live wires L1, L2, and L3 of the three-phase AC power grid 104, respectively.
FIG. 3 is a simplified flowchart diagram illustrating how a three-phase micro-inverter turns off a conversion circuit to improve the conversion efficiency in the prior art. Each three-phase micro-inverter 101 includes three single-phase inverter circuits 102, wherein each single-phase inverter circuit 102 includes two interlaced parallel conversion circuits A1, A2, B1, B2, C1 and C2. As such, each three-phase micro-inverter 101 includes six conversion circuits A1, A2, B1, B2, C1 and C2. Each single-phase inverter circuit 102 has its two conversion circuits operate when the power is high, and turns off one of the two conversion circuits when the power is low, in which case only one conversion circuit functions. For example, a three-phase micro-inverter 101 may have a full power of 200 W, and one conversion circuit is turned off when the operating power is lower than 100 W, thus only one conversion circuit functions, thereby reducing the loss.
FIG. 4 is a simplified schematic diagram of a three-phase micro-inverter which turns off one conversion circuit to reduce loss in the prior art. As shown, each three-phase micro-inverter 101 is coupled to the three-phase AC power grid 104 in the same way. If a three-phase micro-inverter system consists of a group of 3 three-phase micro-inverters 101, the full power P0 for this three-phase micro-inverter system is 600 W. When the system has an operating power less than P0/2, i.e., 300 W, the 3 single-phase inverter circuits 102 each have to turn off one conversion circuit at the same time, e.g., A2, B2, and C2, in order to reach balance among the three phases of the power grid 104. That is, each three-phase micro-inverter 101 has to turn off three conversion circuits in total. Consequently, when the system has an operating power of 300 W, the power ratio is 50%, and the efficiency is equivalent to a single-phase inverter that has an operating power of 100 W. At an even lower power, the efficiency of the three single-phase inverter circuits 102 will decrease similarly as a single-phase inverter circuit 102 with one conversion circuit in operation.
In fact, the operation efficiency of a three-phase micro-inverter varies with the input power, and especially, it drops quickly at low power. For a solar photovoltaic system, the inverters will operate in a low power state for a long duration, due to the environment characteristics. Then, there is a need to simply improve the conversion efficiency of a solar photovoltaic three-phase micro-inverter system while operating in low power.